I agree that the above diagnoses you looked up all cause proteinuria (or protein in the urine). But since you're 15 years old, and I take it that you're pretty active in sports, and feel fine, with no leg or eyelid swelling I presume... and I'm presuming also your doctor checked your blood pressure and it is fine... most likely this is nothing to be concerned about.
I'm thinking your doctor also had you send some blood for tests. There is a specific test called creatinine level that I would look at (creatinine is a molecule from normal muscle breakdown from daily normal activities... this molecule is filtered by the kidneys and not reabsorbed... that is, spilled in the urine as waste product. If this level is elevated in your blood, chances are your kidneys aren't doing a good job in filtering your blood well... and that happens when you're kidneys are sick); if your doctor had it done, take a look at it and see if your level is within normal reference values. If so, then chances are your doctor will only have you repeat another urinalysis in 2-3 months. If the proteinuria is gone, then you're ok. Doctors term this as transient isolated proteinuria... which can happen because of exercise, or because of the thyroid condition you mentioned (a blood test called a TSH can determine at first blush if your thyroid is abnormal or not, and I'm pretty sure your doctor ordered for it) or maybe because you didn't give a "clean midstream catch" of your urine sample (which may have been contaminated with skin cells causing your urine to be positive for protein), or it maybe residual mucus lining the urethra from (?) masturbation [I know you do it... ;) like most if not all other boys your age]
If by any chance your proteinuria persists after repeat urinalysis, your doctor may do what is called a 24-hour urine collection to check if the protein (which is predominantly albumin) in your urine is exceeding 2 grams over 24 hours or not, as well as the so-called creatinine clearance (which is a fancy term to check if your kidneys are in fact working efficiently to clear toxins... or in other words, if they're effectively clearing the creatinine levels in your blood. It's a better measurement than a single random blood creatinine level, but it is cumbersome to do, and it is done only when your initial blood creatinine level is abnormal). Both of these tests require you to collect urine for 24 hours, which can be a pain in the a$$ (keep in mind that some doctors would just opt to check what is called a spot albumin/creatinine ratio of your urine... which has been validated to be nearly if not as accurate as the 24-hour counterpart... but it's every doctor left to do his/her own style). If your urine albumin excretion in 24 hours is less than 2 grams and your creatinine clearance is within normal limits on the background of having proteinuria in your urinalysis, then this is nothing to be worried about... all you need is regular checkups with your doctor. A lot of times, there really isn't a clear understandable cause as to why transient isolated proteinuria occurs, but it happens to normal people. If the urine albumin is more than 2 grams (especially with an abnormal creatinine clearance), your doctor will do some further workup (which I'm not going to discuss anymore), or refer you to a kidney specialist.
I know the above paragraph is super wordy, and I hope I stated it clearly and not make you more confused.
I will not worry about this at this point in time since you sound like a perfectly healthy 15-year-old. There is a high chance this is only transient proteinuria of trivial significance. And because of the fact that you feel fine probably suggests that your blood work and thyroid tests will be fine as well. And also be assured that your doctor is actively looking for answers, and she knows what she is doing and will keep a close watch on it. If she sees something abnormal with your bloodwork, she will act on it.
Hope this helps alleviate most of your worries. Peace out.